In the weekend edition of Le Monde, more precisely in the Sciences section, I read a report on a 2011 study made by Levy et al. who observed that the birthrate drops at Halloween and surges at Valentine’s Day… The above graph illustrates the fact for Halloween, with a significant [meaning?!] 5.3% decrease for spontaneous births. The increase for Valentine’s Day is 3.6% (still for spontaneous births). Even though those data are the result of a survey of all births in the United States over 11 years, thus unlikely to exhibit sampling biases, I am fairly bemused both by the phenomenon and by the interpretation made in the study, namely that “pregnant women may be able to control the timing of spontaneous births” (while I find less astounding that “scheduled births are also influenced by the cultural representations of the two holidays“, even though there may be an administrative bias as well). Being unfamiliar with the U.S. procedure for delivery of birth certificates (and how much both Valentine’s Day and Halloween are of a public holiday), I wonder if this may be a reporting rather than biological bias…